THE FRONTIER LORD BEGINS WITH ZERO SUBJECTS: TALES OF BLUE DIAS AND THE ONIKIN ALNA

After decades of war, Dias returns home as a hero and is rewarded with his own domain. When he arrives at his new lands, however, they turn out to be little more than endless grassy plains without even a single subject. Literally put out to pasture, Dias is stumped as to how to live any life other than a soldier's. But when a horned girl named Alna crosses his path, Dias takes his first steps to being the lord of a not-so-empty frontier. (Source: J-Novel Club) — Description from AniList community (anilist.co).